Molded polycarbonate bodies are often used because of their resistance to shocks and knocks, their tensile strength and transparency. However, the surface properties of polycarbonate materials, such as poor resistance against scratches and wear as well as sensitivity to many solvents and chemicals, considerably limit their use particularly as break-resistant "glass".
A multitude of proposals have been previously offered for obviating these shortcomings, for example the application of coatings based on melamine resins or polysiloxane resins, such as described for example in U.S. Pat. Nos. 3,707,397 and 3,843,390.
However, these coatings have additional disadvantages, aside from difficulties in manufacturing. For example, it is necessary to apply these as lacquers or coatings containing solvents, some of which attack polycarbonates, where cross linking must be performed at higher temperatures. This may negatively affect transparency, as well as the positive properties of consistency of the molded polycarbonate bodies, up to and including breaking of the coated part because of brittleness.
It appears that UV hardening coating systems would be particularly suitable for applying protective coatings onto polycarbonate because of their quick hardening at low temperature. Examples of this approach are set forth in U.S. Pat. No. 3,968,305 and PCT Publication WO 80/00968. However, scratch resistance as well as adhesion to the substrate are not satisfactory in both of these prior systems, and the mechanical properties of the coated part also suffer.
In accordance with German Patent Disclosure DE-A-38 19 627, reaction products of hydroxyl alkyl acrylates with aliphatic polyisocyanates, containing at least two isocyanate groups and at least one biuret group per molecule and/or uretdion, i.e. a cyclo-dimerized isocyanate with two ketone groups, are used for scratch-proof coatings of molded polycarbonate bodies and are hardened by UV radiation. However, the scratch resistance of the surface layer of the polycarbonate parts coated in this manner is not as glassily hard as desired. Additionally, the mechanical strength of the parts coated in accordance with German Patent Disclosure DE-A-38 19 627 is unsatisfactory. Depending on the stress applied, injection molded bodies can break because of brittleness.
German Patent Disclosure DE-A-31 34 157 discloses coating compositions which essentially use greater amounts of N-vinyl derivatives or of cyclic, secondary amides. In this case it is disadvantageous that UV irradiation has to be performed several times to obtain satisfactory scratch resistant surface, such as stated in Example 1 of German Patent Disclosure DE-A-31 34 157. It is only possible to attain low viscosity of the coating solution by the use of larger proportions of monofunctional compounds, such as N-vinyl pyrrolidone. However, if this is done, the scratch resistant surface, which in any event does not have the desired glassy hardness, is further weakened and weathering resistance is also reduced. Added to this is the danger of etching, whitening and triggering of stress cracking of the polycarbonate in the wet state of the coating, i.e. prior to coating hardening.